Magnetic storage systems store information by magnetizing bit positions on tracks located on a surface of a magnetic media, e.g. a magnetic disk. An actuator arm supports and maintains a Magneto-Resistive (MR) head close to the magnetic disk surface to perform the read and write operations on the disk surface. As the magnetic disk is moved past the MR head, the variations in the magnetic flux passing through the MR head result in changes in the electrical resistance of the MR head.
An MR head is biased with a constant electrical current so that a voltage is present across the MR head. A preamplifier is then used to detect changes in the voltage across the MR head caused by the variations in the electrical resistance of the MR head. Changes in the voltage across the MR head are used to extract the data stored on the magnetic disk surface.
FIG. 1 illustrates preamplifier 10A with a single MR head H0, used in the read circuitry of a magnetic storage device, as known in the prior art. The preamplifier 10A measures high frequency variations in the voltage across MR head H0, a variation of approximately 1 mV peak-to-peak. The signal from MR head H0 is applied to the base terminals of transistors Q1 and Q3 which are part of two emitter-coupled differential amplifiers.
Operational amplifiers A1 and A2 bias the transistors of the differential amplifier, Q1-Q8, to operate in the linear region. Capacitors C1 and C2 filter out high frequency signals input to the base terminals of transistors Q2 and Q4.
Nodes N3 and N4 provide a differential output signal for use in an amplifier circuit not shown.
Preamplifier 10A supports 4 to 10 MR heads. FIG. 2 depicts how two MR heads H0 and H1 would be configured in preamplifier 10A. Only one MR head operates at a time. Though not depicted in FIG. 2, a circuit selects an MR head to operate. For example, for MR head H1 to operate, current sources I25 and I26 are turned on, bipolar transistors Q21 and Q23 are biased to operate in the linear region, current sources I1 and I3 are turned off, and transistors Q1 and Q3 do not operate. Transistors Q21 and Q23 essentially take the place of transistors Q1 and Q3 and thus provide inputs to differential amplifiers D1 and D2. Transistors Q2 and Q4 would still operate for transistors Q21 and Q23. Current sources I25 and I26 provide the same level of current as current sources I1 and I3.
In the design of preamplifier 10A, a low noise contribution is paramount. Noise contribution primarily includes transistor noise, which further includes base shot noise and collector shot noise and base thermal noise. Base thermal noise is related by the resistance from the base regions, R.sub.b, an expression commonly known in the art. Increasing the size of transistors Q1-Q4 reduces the magnitude of R.sub.b and thus reduces the base thermal noise. Collector shot noise also contributes to transistor noise. Increasing the gain from base to collector in transistors Q1-Q4 reduces collector shot noise. However, the gain is limited because the transistors must operate in the linear region. Increasing the values of load resistors R1 and R3 increases the gain, but increasing the gain increases the current through resistors R1-R4 and thus may jeopardize linear region biasing of the transistors. Thus the gain is limited and so too is the reduction in collector shot noise. Thus the operation of preamplifier 10A is hampered by noise.